Devices including bundles of hollow fibers of the type described above have been used, for example, for various types of medical treatments, such as hemodialysis, hemofiltration, plasmapheresis, and immunotherapy. In addition, such devices have been used in other fields, such as in the general fields of dialysis and of filtration as in connection with the cleaning of sea water.
In connection with the production of these types of devices, both ends of the housing containing the bundle of fibers therein are normally enclosed in a casting mold, into which the material for the respective end wall is fed in a liquid condition for penetration into and around the fibers. It is necessary to control that penetration in order that the degree of such penetration into the fibers is less than the degree of such penetration around the fibers, thereby making it possible to open the interior of the fibers by making a cut at a location between the planes represented by these two different degrees of penetration.
Such control is obtained in a variety of ways. According to one such known method, the ends of the fibers are first closed by means of a special material, such as by means of a so-called "hot melt". Thus, subsequent to this closing, the end wall is itself cast in a second step.
In an alternative known method, the two end walls are molded at the same time in a manner such that the volume of air which is enclosed within the fibers is compressed, thereby providing an increase in the pressure therein, which in turn thereby prevents further penetration.
In order to further control the degree of penetration of this material in and around the fibers, such methods are often combined with the application of rotational movement around an axis essentially perpendicular to the londitudinal axis of the housing. By the use of such movement, the centrifugal forces thus created influence the depth of penetration both within and around the fibers.
A more detailed description of such centrifugal casting can be found, for example, in U.S. Pat. Nos. 3,339,341; 3,442,002; 3,492,698 and 3,499,062.
Another method for providing uniform penetration by utilizing two liquids in combination with a compressed gas is described in U.S. Pat. Nos. 4,049,765 and 4,105,731. According to this method, a bundle of fibers is dipped into a solidifyable liquid, whose upper surface is controlled by a second liquid, which might in turn be exposed to the pressure of a compressed gas.
In spite of these known control methods, however, it has been difficult, for example, to provide dialysers in a reproduceable way, and within narrow tolerance limits. Furthermore, it has been necessary to permit a relatively great penetration into the fibers, and because of this penetration a relatively large waste of both casting material and fiber material has occurred when the fibers are then opened.